Device for thermosurgery

ABSTRACT

An apparatus for the thermosurgical treatment of biological tissue comprises a generator ( 22 ) for providing treatment energy, measuring means ( 24 ) for registering the temporal progression of a measured quantity that is influenced by the tissue impedance of the treated body or representative of the tissue impedance, and also a control unit ( 26 ) which has been set up to ascertain a frequency spectrum within a predetermined examination frequency range for the temporal progression of the measured quantity and to control the energy output of the generator ( 22 ) in a manner depending on the spectral content of the measured quantity within the examination frequency range. The invention is based on the finding that the genesis of vapour bubbles in the heated tissue can be detected on the basis of the frequency spectrum of the tissue impedance, in particular within a frequency range between approximately 0.5 kHz and 200 Hz. Depending on the extent of the vapour bubbling, the tissue impedance displays a differing spectral image within this frequency range. In accordance with the invention, this finding is utilised for the purpose of controlling the energy output of the generator.

The invention relates to an apparatus for thermal surgery.

In thermal surgery biological tissue is heated by introduction oftreatment energy, in order to achieve a defined therapeutic aim bydenaturation of the treated tissue. In particular, in thermal surgery acoagulation or ablation of local tissue regions is striven for, forexample on the inner cardiac wall of the heart chambers, on the valvesof the heart chambers, on the veins and arteries connected to the heart,or on other blood-conveying vessels of the human or animal body. Inrespect of the heart, cases of arrhythmia or tachycardia, for example,can be treated by means of thermal surgery. It will be understood thatthe field of application of the thermosurgical apparatus according tothe invention is not restricted to cardiac treatments. In principle, theapparatus according to the invention is suitable for the thermosurgicaltreatment of arbitrary sites on or within the body.

Various methods are known for the introduction of the treatment energy,including application of a high-frequency a.c. voltage, oscillating as arule within the two-digit or three-digit kHz range, via electrodesattached to the body (one then speaks of HF surgery), or irradiationwith ultrasound, laser light or microwaves. Again, in principle theapparatus according to the invention is not subject to any restrictionas regards the type of the generation of heat in the treated tissue, beit electrical or by means of other active mechanisms.

In the course of thermal treatment of biological tissue with the aim ofthe denaturation thereof it is known that the impedance of the tissuechanges with the tissue temperature. In particular, it is known that thetissue impedance firstly decreases with increasing heating, before thelowering of the impedance is attenuated and the curve of the impedancecharacteristic becomes flat, but that the tissue impedance then oftenshoots up jerkily. In order to avoid undesirable charring or scabbing ofthe tissue, frequently the aim is to control the input of energy intothe tissue in such a way that the tissue impedance remains within theregion of its declining or flattened characteristic branch but in nocase is the strongly ascending branch of the impedance characteristicattained. For this purpose it is known to measure the tissue impedancecontinuously and, on the basis of the measured impedance, to controlmanually or automatically the energy output of a generator providing thetreatment energy. In DE 31 20 102 C2, for example, the derivative of thetemporal progression of the tissue impedance is ascertained, and thepower of the generator is reduced if the derivative attains a valueclose to zero. It has certainly been shown that for many applicationseven this method is not sufficiently reliable for controlling the powerof the generator.

It is therefore the object of the invention to create an apparatus forthermal surgery that is able to avoid undesirable charring or scabbingof the treated tissue reliably.

With a view to achieving this object, an apparatus according to theinvention for thermal surgery includes a generator for providingtreatment energy, measuring means for registering the temporalprogression of a measured quantity that is influenced by the tissueimpedance of the treated body or that is representative of the tissueimpedance, and also a control unit which has been set up to ascertain afrequency spectrum within a predetermined examination frequency rangefor the temporal progression of the measured quantity and to control theenergy output of the generator in a manner depending on the spectralcontent of the measured quantity within the examination frequency range.

The finding underlying the invention is that in biological tissue, whenit is heated, microscopically small bubbles arise which find expressionin the tissue impedance by virtue of a characteristic spectrum varyingin temperature-dependent manner. By ascertainment and evaluation of thefrequency spectrum of the tissue impedance, in this way the currenttemperature in the treated tissue can be inferred, and, dependingthereon, the power of the generator can be controlled. Since, prior to acharring or scabbing of the tissue, bubbling can firstly always bedetected within the tissue or on the surfaces of applied electrodes(human or animal tissue consists, to a not insignificant extent, of anaqueous saliferous solution, the water contained therein evaporatingupon heating and turning into vapour bubbles), by suitable control ofthe power of the generator upon appearance of the bubbles undesirablysevere damage to the s tissue can be safely avoided. Via the frequencyspectrum of the tissue impedance or, expressed generally, of a measuredquantity influenced by the tissue impedance or representing the latter,the bubbling and the intensity thereof can be reliably detected.

It has been shown that the bubbles arising upon heating of the tissuedisappear more or less quickly. Within a few fractions of a second theymay cool again so considerably on account of the cooling effect byvirtue of the colder environment that they dissolve. It has furthermorebeen shown that with increasing input of energy into the tissue and withincreasing heating of the tissue the size of the is vapour bubbles andtheir lifespan (residence-time) increase, but simultaneously the numberof bubbles arising per unit of time (frequency) decreases.

In particular, the following relationship was observed: at comparativelylow temperature many small bubbles with comparatively short lifespanarise; at medium temperature moderately many bubbles with medium sizeand medium lifespan arise; at comparatively high temperatures, on theother hand, comparatively few bubbles with large size and long lifespanarise. As a rule of thumb, the following was accordingly established:the bubble size and the lifespan are proportional to the temperature,whereas the frequency of occurrence of the bubbles is inverselyproportional to the temperature. It was further observed that thetemperature of the onset of bubbling lies, as a rule, above 40° C.

A direct optical or other registration of these fine bubbles is notpossible, or only possible with difficulty, with present-day medicalinstruments. It has to be considered that many interventions take placewithout a view of the site of the operation, for example in the closedheart chamber or in deep regions of the brain.

The invention takes advantage of the fact that the vapour bubblesinfluence the tissue impedance. In concrete terms, they becomenoticeable through fluctuations in tissue impedance. These fluctuationscan be registered via a spectral analysis of a measured quantitydepending on the tissue impedance. It has been shown in experiments thatthe changes in tissue impedance caused by bubbles typically exhibitfrequencies between 80 Hz down to below 1 Hz. The higher frequencieswere observed at temperatures around 40° C. to 50° C. Since many smallbubbles with short lifespan arise at these comparatively lowtemperatures, the resulting fluctuations in impedance have comparativelysmall amplitude but arise at comparatively high speed. The lowfrequencies down to below 1 Hz, on the other hand, were predominantlyobserved at comparatively high temperatures between 80° C. and 100° C.At these temperatures, large bubbles with long lifespan predominate,which bring about slow but comparatively strong changes in tissueimpedance. In particular, it was established that the fluctuations intissue impedance at the high temperatures may be greater than at the lowtemperatures by a two-digit or even three-digit factor, for example 200times to 500 times as great.

The control unit of the thermosurgical apparatus according to theinvention utilises the above finding of a relationship between thetissue temperature and the spectrum of the tissue impedance bycontrolling the energy output of the generator in a manner depending onthe spectral content of the analysed measured quantity within apredetermined examination frequency range. The upper limit of theexamination frequency range preferentially lies at most at approximately5 kHz and at least at approximately 80 Hz, preferentially at least atapproximately 100 Hz, and most preferentially at least at approximately200 Hz. The lower limit of the examination frequency range, on the otherhand, preferentially lies at most at approximately 2 Hz and at least atapproximately 1 Hz, preferentially at least at approximately 0.5 Hz, andmost preferentially at least at approximately 0.1 Hz.

Within the examination frequency range evaluated by the control unit itcannot be ruled out that, besides the spectral components caused bybubbles, other components arise that are to be attributed to othercauses. In particular, components that are caused by the cardiacactivity may be superimposed on the spectral components caused bybubbles. However, such spectral components caused by the heartbeat ariseonly periodically. By time-periodic filtering (blanking) of the signalof the measured quantity, the spectral components caused by theheartbeat can be removed from the frequency spectrum of the measuredquantity.

Depending on the technology that is employed (e.g. calculation by meansof discrete Fourier analysis, analogue filter banks or such like), thefrequency spectrum can be re-ascertained by the control unit incontinuous-time manner or at regular, sufficiently short time-intervals.The current frequency spectrum in each instance is then evaluated by thecontrol unit and—if necessary—converted into corresponding controlcommands for the generator, in order to raise or lower its outputenergy. For the evaluation of the frequency spectrum, the control unitmay have been set up to ascertain from the frequency spectrum at leastone characteristic spectral parameter of the frequency spectrum of themeasured quantity, for instance a characteristic frequency or acharacteristic amplitude. For example, a characteristic frequency may bethat frequency at which the frequency spectrum within the examinationfrequency range has a (local or global) amplitude maximum. Acharacteristic amplitude may be, for example, the amplitude at such a(local or global) amplitude maximum of the spectrum.

From the above elucidations relating to the temperature dependence ofthe bubbling process it can be seen that a characteristic frequencydefined in such a way will shift towards smaller frequency values in thecase of rising tissue temperature, whereas simultaneously the maximumamplitude of the frequency spectrum will become increasingly larger.Therefore the control unit may expediently have been set up to lower thepower of the generator during a treatment procedure in response to adiminution of the value of the characteristic frequency or/and inresponse to an increase in the value of the characteristic amplitude.

It will be understood that other definitions of a characteristicfrequency and a characteristic amplitude, or generally of the at leastone characteristic spectral parameter, are possible. For example, acharacteristic frequency could be defined as the centre frequency of adefined frequency range of the ascertained frequency spectrum, forinstance of a frequency range within which the frequency spectrum liesabove a predetermined amplitude threshold. A characteristic amplitudecould be defined, for example, as an amplitude average that is obtainedby weighted or unweighted averaging of the spectral amplitude within adefined frequency range of the ascertained frequency spectrum.

It is also conceivable to ascertain several differently definedcharacteristic frequencies or/and several differently definedcharacteristic amplitudes from the frequency spectrum of the measuredquantity and to undertake the control of the energy output of thegenerator on the basis of each of the characteristic frequencies or eachof the characteristic amplitudes. Expediently the at least onecharacteristic spectral parameter should be defined in such a way thatit (or they, if several characteristic spectral parameters are defined)displays a pattern that is variable in temperature-dependent manner andthat is a measure of the tissue temperature.

In one embodiment of the invention, the control unit has been set up toderive, in a manner depending on a plurality of input parameters, atleast some of which are characteristic of the ascertained frequencyspectrum, an auxiliary parameter that is representative of the tissuetemperature, and to control the energy output of the generator in amanner depending on the value of the auxiliary parameter. In thisconnection the control unit for the derivation of the auxiliaryparameter may expediently access stored information via a relationship,ascertained in advance, between the input parameters and the tissuetemperature. This relationship may, for example, be obtained in advanceby empirical methods through experiment. The stored information mayexpress the relationship functionally in the form of an algorithm, forexample. It may also express it in the form of a table or a set oftables. In a preferred embodiment, the input parameters include both acharacteristic frequency and a characteristic amplitude of theascertained frequency spectrum.

For the energy output of the generator, a regulation that is continuousat least in time segments is conceivable, in the course of which thecontrol unit adjusts the energy output of the generator to apredetermined or predeterminable desired value or to a desired-valueprofile of the auxiliary parameter.

Alternatively or additionally, it is conceivable that the control unithas been set up to change the energy output of the generator in a mannerdepending on the crossing of at least one predetermined threshold of theauxiliary parameter, in particular in stepped manner. For example, itmay be the case that the control unit switches the generator off atleast temporarily if a limit temperature of the tissue indicated by acorresponding threshold is exceeded.

It has been shown that the frequency spectrum of the measured quantitymay depend on further influencing factors in addition to the tissuetemperature. The input parameters from which the control unit ascertainsthe auxiliary parameter may therefore include, besides the at least onecharacteristic spectral parameter of the ascertained frequency spectrum,additionally one or more further parameters that are representative ofthe further influencing factors mentioned above and that are not derivedfrom the ascertained spectrum. One of the influencing factors may be, inparticular, the air pressure.

It has been shown that the prevailing air pressure may have aconsiderable influence on the bubbling and hence on the impedancespectrum. It is known that the partial pressure of the vapour bubbles inthe biological tissue (e.g. heart chamber or blood vessels) changes withthe external pressure (air pressure). A high external pressure shiftsthe temperature threshold, starting from which the bubbling sets in,upwards. A lower external pressure, on the other hand, permits vapourbubbling already at significantly lower temperatures. With a view tomore precise determination of the tissue temperature and, as a result,of the size and depth of the heated tissue region, it has thereforeproved expedient to take account of the air pressure in connection withthe control of the energy output of the generator. For example, for thispurpose the control unit may receive from a pressure sensor a sensorsignal that is representative of the measured air pressure. It is alsoconceivable that the user can set an air-pressure value on the apparatusmanually and that the control unit draws upon the air-pressure valuepredetermined by the user.

The consideration of those input parameters which the control unit doesnot derive from the ascertained frequency spectrum but receives in someother way (by sensory measurement, user default) in connection with thecontrol of the generator may, for example, be effected in such a mannerthat the control unit ascertains from the at least one spectrum-relatedinput parameter firstly a preliminary value of the auxiliary parameterand then corrects this preliminary value multiplicatively or additivelyby means of one or more correction factors, in order in this way toarrive at a definitive value of the auxiliary parameter. The controlunit can take the value of the respective correction factor from, forexample, a correction table ascertained and stored in advance. Such acorrection table is to be understood as part of the aforementionedinformation concerning the relationship between the input parameters andthe tissue temperature.

Correction factors may, for example, also have been provided forinfluencing factors such as the type of tissue or such as the locationand/or type of an electrode via which an electrical signal that is drawnupon for the ascertainment of the measured quantity is tapped on thebody. For instance, it has been shown that the vapour bubbles may behavedifferently in tissue that is intensely supplied with blood than incomparatively hard tissue layers. Furthermore, it has been shown thatdifferent measured values may result at skin electrodes or surfaceelectrodes than at passive measuring electrodes for electrophysiologicalsignals or at active high-frequency electrodes with which an electricala.c. voltage is applied to the tissue in the course of the HF surgery.

According to an advantageous further development of the invention, thecontrol unit may have been set up to present the ascertained frequencyspectrum or at least one quantity derived therefrom on a display unitfor visual representation. This permits the operating surgeon a personalexamination of the course of the treatment and, where appropriate, anadaptation of the output power of the generator via suitable operatingelements on the apparatus according to the invention.

The invention will be elucidated further in the following on the basisof the appended drawings. Represented are:

FIG. 1: an exemplary temporal progression, to be understood purelyqualitatively, of a voltage-measuring signal in which fluctuationsbecome evident which are to be attributed to the formation of vapourbubbles in the tissue during a thermosurgical application,

FIG. 2: schematically and again purely qualitatively, the changingfrequency spectrum of the tissue impedance in the case of increasingheating of the tissue,

FIG. 3: schematically, a block diagram of an exemplary embodiment of athermosurgical apparatus according to the invention and

FIG. 4: components of a measuring amplifier of the apparatus shown inFIG. 3.

FIG. 1 shows a typical electrophysiological voltage signal such as canbe tapped on a living body. For example, the voltage signal may betapped at the application part of a cardiac catheter, at internal leadelectrodes of such a cardiac catheter, at ECG electrodes or referenceelectrodes attached to the body externally or at a counter-electrode.The diagram shown in FIG. 1 clarifies the fluctuations of the voltageamplitude which are observable in the course of the thermosurgicaltreatment of the body, caused by vapour bubbling in the tissue and,associated therewith, by a change in tissue impedance. It is to bestressed once again that the representation in FIG. 1 is to beunderstood purely qualitatively; in particular, no inferences of theactual ratios are to be deduced from the represented ratio of theintensity of the voltage fluctuations caused by bubbles to the baselevel of the voltage signal. In practice, the voltage fluctuationscaused by bubbles may be small in comparison with the base level of thevoltage-measuring signal.

Identified by arrows in the diagram shown in FIG. 1 are a total of fivedifferent phases, which are associated with differing degrees of heatingof the tissue and hence with differing degrees of vapour bubbling. Phase1 labels the start of the input of energy into the tissue; as yet, noexcursions in the voltage-measuring signal can be discerned. However,vapour bubbling sets in already upon slight heating of the tissue. Itbecomes noticeable in the voltage-measuring signal through comparativelyweak voltage fluctuations. The fluctuations arise relative quickly, i.e.they have a relatively high frequency. This is phase 2 in the diagramshown in FIG. 1.

Upon further increase of the temperature of the tissue the voltagefluctuations become steadily greater. At the same time, the speed ofthese artefacts decreases. In the frequency spectrum this becomesnoticeable through correspondingly lower frequencies with simultaneouslyhigher spectral amplitude. Phases 3 and 4 in the diagram shown in FIG. 1clarify this increase in the intensity of fluctuation withsimultaneously decreasing frequency of the changes in voltage.

Upon intense heating of the tissue, the voltage fluctuations arising aregreater still. The amplitude of these voltage fluctuations reaches itsmaximum. At the same time, the frequency of the voltage fluctuationsfalls greatly.

FIG. 2 clarifies, again purely qualitatively, the associated spectrum ofthe voltage signal within the frequency range that is relevant to vapourbubbling. This frequency range ranges, for example, from 0.5 Hz toapproximately 200 Hz, although the upper and lower limits of thefrequency range investigated may, of course, be chosen differently inthe concrete application. The spectrum of the changes in tissueimpedance brought about by vapour bubbling, and hence in the voltagefluctuations of the measuring signal upon slight heating of the tissue,is denoted in FIG. 2 by S₁, whereas the spectrum upon intense heating isdenoted by S₂ and, in contrast to the spectrum upon slight heating, isonly indicated by dashes.

The spectral envelopes sketched in FIG. 2 are, of course, to beunderstood as being exemplary only; in the concrete application thespectra may display a different shape. What is important is merely thefact that with increasing heating a shift of the spectrum towards lowerfrequencies takes place, while the spectral amplitude simultaneouslybecomes larger. This behaviour is represented in FIG. 2 by a shift arrowP pointing towards lower frequencies and towards larger spectralamplitudes.

The temperature-dependent shift of the tissue-impedance spectrum isexploited in accordance with the invention in order to control the inputof energy into the tissue. In particular, the power of the generator canbe cut back before the spectrum drifts too far towards low frequencies,in order in this way to avoid burns and scabbing of the tissue.

It has been shown that for the identification of the spectral content ofthe tissue-impedance spectrum two defined spectral parameters areparticularly suitable, namely the frequency of greatest amplitude(labelled in FIG. 2 by f₀) and also the maximum spectral amplitude(labelled by A₀). From the above elucidations it can readily be seenthat these two parameters change with increasing heating of the tissue:whereas the value of the frequency f₀ decreases, the value of theamplitude A₀ becomes greater. With given boundary conditions, the valuepair (f₀/A₀) is a reliable indicator of the current tissue temperature.The corresponding relationship between these spectral parameters and thetissue temperature can be ascertained experimentally, for example, andrecorded in characteristic form or in the form of a functionalexpression. It should be pointed out that it is generally not excludedto use solely the spectral parameter f₀ or solely the spectral parameterA₀ (where appropriate in conjunction with further influencing parametersthat are independent of the spectrum of the tissue impedance) inconnection with the control of the input of energy into the tissue. Foreach of the named spectral parameters alone also displays acharacteristic behaviour that is dependent on the tissue temperature.

In the block diagram illustrated in FIG. 3 a biological tissue 20 isindicated, into which a coagulation instrument or ablation instrument,not represented in any detail, is introduced. Via the instrument and anelectrode arrangement provided on the instrument, an electrical a.c.voltage is applied to the tissue 20 by a high-frequency generator 22.Typically the frequency of the a.c. voltage employed for the tissuetreatment lies within the three-digit kHz range right up to theone-digit MHz range. For example, it amounts to approximately 200 kHz orapproximately 500 kHz. In the case of bipolar instruments the treatmentalternating voltage is applied between two electrodes attached to thetip of the instrument that has been introduced; in the case of monopolarinstruments only the application electrode is located at the tip of theinstrument; the counter-electrode is located externally on the surfaceof the treated body. The invention is generally independent of the typeof instrument used and also independent of the type of the introducedtreatment energy, be it electrical energy, electromagnetic energy,optical energy or acoustic energy.

For the purpose of registering the vapour bubbling, which arises as aconsequence of the increasing heating of the treated tissue, use is madeof transducers 24 which in each instance tap an electrical measurementvoltage via an electrode attached to the body internally or externally,subject said measurement voltage to band-pass filtering and amplify itand supply the measured signal obtained in this way to an electroniccontrol unit 26. In the exemplary case of FIG. 2 a total of fourtransducers 24 are shown. In each case a transducer 24 is preferentiallyconnected at least to each of the active HF electrodes via which thetreatment alternating voltage of the generator 22 is fed into the tissue20 (application electrode as well as indifferent or neutral electrode);these transducers are, as a rule, the primary locations of the evolutionof heat. The number of transducers 24 is, of course, variable at anytime. It may be sufficient to tap only a single measurement voltage onthe body. In many applications, however, it will be expedient to tapmeasurement voltages at various places on the body (internally or/andexternally), in order in this way to be able to detect instances ofpossible local overheating of the tissue in different regions of thebody. In this connection it should be pointed out that externalelectrodes that have not been attached properly may result in locallyincreased current densities and, in the worst case, undesirable burns.Such instances of undesirable overheating of the tissue can also beprevented with the thermosurgical apparatus according to the invention,by the power of the generator being already brought down upon theoccurrence of slight bubbling.

According to FIG. 4, each of the transducers 24 exhibits by way offunctional components a low-pass filter 28, a high-pass filter 30 andalso a subsequent amplification module 32. The two filters 28, 30together bring about a band-pass filtering of the measured signal, thepass band being, for example, between approximately 0.5 Hz andapproximately 200 Hz. Within this range the relevant portion of thespectral components associated with the formation of vapour arises,according to the prior state of knowledge.

The control unit 26 then ascertains a frequency spectrum for each of theband-pass-filtered and amplified measured signals, for example byFourier analysis or other suitable spectral examination methods. Foreach frequency spectrum ascertained in this way it ascertains thecurrent values of the spectral parameters f₀ and A₀ and ascertains, in amanner depending thereon, an auxiliary parameter which is a measure ofthe intensity of the vapour bubbling that has had an influence on themeasured signal in question, and hence a measure of the tissuetemperature in the region of that place where the measured signal inquestion was tapped on the body. For the purpose of ascertaining theauxiliary parameter, the control unit 26 may have recourse to furtherinput parameters, in particular a current value of the air pressuresupplied by a pressure sensor 34. A memory 36 connected to the controlunit 26 contains information, stored in tabular or algorithmic form,concerning the relationship between the auxiliary parameter and all theinput parameters, including the spectral parameters f₀, A₀ and the airpressure. Depending on the auxiliary parameter ascertained in this way,the control unit 26 then controls the output power of the generator 22in accordance with a control program. For example, for this purpose adesired-value profile of the auxiliary parameter, suitable for therespective treatment, may have been saved in the memory 36, in responseto which the control unit 26 adjusts the auxiliary parameter by powerregulation of the generator 22. Alternatively or additionally, one ormore thresholds may have been stored in the memory 36, and the controlunit 26 may change the power of the generator in stepped manner whensaid thresholds are fallen short of or exceeded. It will be understoodthat the concrete control method may depend on the type of therespective treatment.

As an alternative or in addition to the automatic power regulation ofthe generator 22 outlined above, the thermosurgical apparatus may beopen to a manual generator control. For this purpose a display unit 38on which the control unit 26 can bring about a graphical or numericaldisplay of the ascertained frequency spectrum or at least of thecharacteristic spectral parameters of the spectrum may have beenconnected to the control unit 26. It is also conceivable that thecontrol unit 26 can bring about on the display unit 38 the display of atemperature measure derived from the ascertained frequency spectrum and,where appropriate, from further input parameters, which isrepresentative of the estimated tissue temperature. On the basis of theinformation displayed by the display unit 38, the user can then adaptthe power of the generator 22 manually himself via suitable operatingelements (not represented in any detail). Such a configuration of thethermosurgical apparatus according to the invention (that is to say,with display of suitable information on the display unit 38 for thepurpose of enabling a manual power regulation of the generator insteadof an automatic power regulation) is reserved here expressly as possiblesubject-matter of an application for protection that is yet to beformulated.

The thermosurgical apparatus shown in FIG. 3 may additionally include asource of constant voltage or of constant current (not represented),which provides a stabilised d.c. voltage or a stabilised direct current,respectively. This d.c. voltage or this direct current is fed into thetissue 20. In this way it can be ensured that in the event of a possiblesubtraction of all electrochemical voltages of the body nevertheless asufficient voltage swing for the voltage fluctuations caused by vapourbubbles is possible. Expediently this external voltage supply or currentsupply is such that it has no electrophysiological effects on thebiological tissue.

According to another embodiment variant, the thermosurgical apparatusmay include an a.c. source (likewise not represented) which provides aconstant alternating current, the frequency of which is different fromthe frequency of the treatment a.c. voltage of the generator 22. Thisalternating current is fed into the tissue 20. On the body an a.c.voltage can then be tapped, the amplitude of which is modulated inaccordance with the changes in tissue impedance caused by bubbles. Byenvelope demodulation of the a.c. voltage tapped in this way, a measuredsignal that is representative of the tissue impedance can be obtainedwhich can subsequently be subjected to a spectral analysis in the mannerdescribed above.

Generally, the frequency of this additional “measuring alternatingcurrent” fed into the tissue for impedance-measuring purposes may bechosen within a wide range, so long as it is sufficiently different fromthe frequency of the treatment alternating voltage of the generator 22in order that the voltage response of the body to the excitation by themeasuring alternating current can be discriminated from the treatmentalternating voltage. Accordingly, the frequency of the measuringalternating current may be chosen, for example, within a range thatranges from approximately 5 kHz up to approximately 10 MHz. By way oftypical frequencies of the measuring alternating current, 50 kHz or 100kHz, for example, may be mentioned. The frequency of the measuringalternating current may be lower or higher than the frequency of thetreatment alternating voltage.

By way of a further possible measured quantity which is influenced bythe tissue impedance, the phase shift between the measuring alternatingcurrent fed in and the tapped alternating-voltage response may be drawnupon. Depending on the value of the tissue impedance, the phase positionof the two oscillations relative to one another changes. This can beascertained by the control unit 26 and likewise evaluated spectrally.

According to a further variant, the tissue impedance could beascertained directly from the quantities constituted by voltage andcurrent of the HF energy output by the generator 22 (e.g. ratio of therms values, relative phase position). In this case, a separate measuringalternating current can be dispensed with, this having a favourableeffect on the costs of the thermosurgical apparatus.

1-14. (canceled)
 15. Apparatus for thermal surgery, comprising: agenerator for providing treatment energy; a measuring device forregistering a temporal progression of a measured quantity that isinfluenced by the tissue impedance of a treated body or representativeof the tissue impedance; and a control unit for determining, based onthe measured quantity, a frequency spectrum within a predeterminedexamination frequency range, the frequency spectrum representative of atemporal progression of the tissue impedance, and for controlling theenergy output of the generator based on the determined frequencyspectrum, wherein an upper limit of the examination frequency range liesat most at approximately 5 kHz.
 16. Apparatus according to claim 1,wherein the upper limit of the examination frequency range lies at leastat approximately 80 Hz.
 17. Apparatus according to claim 1, wherein theupper limit of the examination frequency range lies at least atapproximately 100 Hz.
 18. Apparatus according to claim 1, wherein theupper limit of the examination frequency range lies at least atapproximately 200 Hz.
 19. Apparatus according to claim 1, wherein alower limit of the examination frequency range lies at most atapproximately 2 Hz.
 20. Apparatus according to claim 1, wherein a lowerlimit of the examination frequency range lies at least at approximately1 Hz.
 21. Apparatus according to claim 1, wherein a lower limit of theexamination frequency range lies at least at approximately 0.5 Hz. 22.Apparatus according to claim 1, wherein a lower limit of the examinationfrequency range lies at least at approximately 0.1 Hz.
 23. Apparatusaccording to claim 1, wherein the control unit is adapted to determineone or more characteristic spectral parameters from the frequencyspectrum and to control the energy output of the generator based on atleast one of the one or more characteristic spectral parameters. 24.Apparatus according to claim 23, wherein the one or more characteristicspectral parameters include a characteristic frequency of the frequencyspectrum.
 25. Apparatus according to claim 24, wherein thecharacteristic frequency is a frequency at which the frequency spectrumpossesses an amplitude maximum.
 26. Apparatus according to claim 24,wherein the control unit is adapted to lower an output power of thegenerator during a treatment procedure in response to a diminution ofthe characteristic frequency.
 27. Apparatus according claims 23, whereinthe one or more characteristic spectral parameters include acharacteristic amplitude of the frequency spectrum.
 28. Apparatusaccording to claim 27, wherein the characteristic amplitude is a maximumamplitude of the frequency spectrum.
 29. Apparatus according to claim28, wherein the control unit is adapted to lower an output power of thegenerator during a treatment procedure in response to an increase of thecharacteristic amplitude.
 30. Apparatus according to claim 1, whereinthe control unit is adapted to derive, based on a plurality of inputparameters, at least some of which are characteristic of the frequencyspectrum, an auxiliary parameter that is representative of a tissuetemperature and to control the energy output of the generator based onthe auxiliary parameter.
 31. Apparatus according to claim 30, whereinfor the derivation of the auxiliary parameter the control unit isadapted to access stored information concerning a relationship,ascertained in advance, between the input parameters and a tissuetemperature.
 32. Apparatus according to claim 30, wherein the controlunit is adapted to regulate the energy output of the generator based onat least one target value of the auxiliary parameter.
 33. Apparatusaccording to claim 30, wherein the control unit is adapted to change theenergy output of the generator such as in a stepped manner, in responseto at least one predetermined threshold of the auxiliary parameter beingreached.
 34. Apparatus according to claim 30 wherein one of the inputparameters is representative of an ambient air pressure.
 35. Apparatusaccording to claim 1 wherein the control unit is adapted to present atleast one of the frequency spectrum and at least one quantity derivedtherefrom on a display unit for visual representation.